Northern Hemispheric Flow Indices

1. Northern Hemispheric Flow Indices Knight & Grumm NWS-PSU 485 Lecture Series

2. Annular Modes • A different view of the zonal index • global sea level pressure at 55N/S • Northern (NAM) and Southern (SAM) • positive index: westerlies are strong • negative index: more meridional flow • averaged by day, week, month, season • most significant in the winter (NH)

3. Annular Mode • NAM averaged for Jan-Feb-Mar (high) • NAM averaged for Jan-Feb-Mar (low)

4. Annular Mode • NAM averaged Jan-Feb-Mar (high) • NAM averaged Jan-Feb-Mar (low)

5. Annular Mode • Blue shading indicates ratio of cold days (T<1.5sd) to low values of NAM

6. Annular Mode

7. Annular Mode • The Polar Night Jet (PNJ) oscillates in the stratosphere • develops each autumn at about 20km and reaches speeds of 50m/s • Northern Hemisphere PNJ is asymmetric due to thermal contrasts of land/oceans • PNJ is perturbed by upward propagating velocity waves (due to baroclinic waves)

8. Annular Mode • PNJ Dissipation • In Northern Hemisphere, deformation of the polar vortex plus upward propagating planetary waves leads to rapid weakening of the jet. • lower polar stratosphere can warm by 50C as upper stratospheric air replaces cold core jet. • Warmth can propagate to the surface at the high latitudes. (according to O’Lenic – January 2004)

9. Annular Mode • Stratosphere Temperatures - Feb 29 2004

10. Annular Mode • Global Warming and the NAM • Many GCM predict warming in troposphere and cooling in stratosphere • This causes a steep thermal gradient in the lower stratosphere due to tilted tropopause • In turn, leads to stronger than average PNJ which locks coldest air near the Pole • Explains warmth of most winters since 1980’s and patches of frigid air in far north

11. Regional Flow Indices • Indices that affect the flow and climatology over North America and Eurasia • North Atlantic Oscillation (NAO) • Pacific North American pattern (PNA) • Eastern Pacific Index (EP) • Definitions • Impacts on weather • …and a bit of history…..

12. North Atlantic Oscillation • A measure of the flow over the Atlantic Basin • Typically, the value is computed using MSLP departures • over Iceland and the Azores • high or positive NAO associated with deep Icelandic low • low or negative NAO associated with ridge over Iceland and Norway • Periods of low NAO have been linked to cold times in Northern Europe and North America • the Little Ice Age ~1300-1860

13. North Atlantic Oscillation • Monthly and Seasonal, raw and normalized, indices of the NAO are based on the difference in sea level pressure between Ponta Delgada, Azores (38°N, 26°W) and Akureyri, Iceland (66°N, 18°W) from 1874-2000. Raw indices are obtained as the simple monthly sea level pressure anomaly difference between the two stations, biased toward the Arctic where the standard deviation in pressures is approximately twice that of the subtropical Azores location

14. NAOhttp://www.cru.uea.ac.uk/tiempo/floor2/data/nao.htmNAO • Strictly the index should only be applied in the cold season • Last period of low NAO: the winter of 1995-96 • Historically: • NAO did fluctuate more in the past than recent times • We know this from history: • wine harvest dates (later was a cold year) • wheat harvests and periods of European Famine • wheat and other grains failed or produced bad crops in cold wet years. • shipping and fishing industries back to Nordic and Basque fishing times. • they followed the water-temperature sensitive Cod fish

15. NAO-Positive Phase • A stronger than usual subtropical high pressure center and a deeper than normal Icelandic low • The increased pressure difference results in more numerous and stronger winter storms crossing the Atlantic Ocean on a more northerly track. • Wetter in western Europe • Keeps cold air out of Europe • North American Weather • Cold and dry winters in northern Canada and Greenland • The eastern US experiences mild and wet winter conditions

16. NAO-Positive Phase

17. NAO-Negative Phase • Shows a weak subtropical high and a weak Icelandic low. • A reduced pressure gradient results in fewer and weaker winter storms crossing on a more west-east path. • Moist Atlantic air moves into the Mediterranean • Cold air to northern Europe • Associated with many record cold European winters • US east coast experiences more cold air outbreaks and snowy weather conditions. • Greenland, has milder winter temperatures • during the warm conditions 800-1250, negative NAOs may have allowed farming and fishing in Greenland

18. NAO-Negative Phase

19. NAO in recent history

20. NAO by Seasons

21. Annual NAO Values 00 01 02 03 04 05 06 07 08 09 1860 -0.66 -0.20 -3.04 4.14 0.42 1870 -2.77 -0.85 -0.83 0.17 2.32 -2.10 -1.85 -0.24 -4.03 -0.69 1880 -1.86 -2.84 2.06 1.73 1.38 -1.40 0.20 -3.05 -2.94 2.57 1890 2.97 -0.34 -2.49 -1.32 2.28 -3.41 1.58 0.38 0.95 -2.40 1900 0.22 -0.78 -2.61 1.69 1.46 1.12 2.29 2.36 2.51 -0.87 1910 1.62 0.99 0.11 2.85 2.40 -2.66 -0.60 -2.21 0.45 -0.47 1920 2.97 2.32 2.21 2.43 -0.02 0.18 -0.31 -0.49 1.43 -0.51 1930 1.46 -0.48 -0.60 -0.21 1.91 1.90 -1.00 -0.81 3.75 -0.66 1940 -1.78 -2.10 -0.84 3.31 0.71 -1.69 1.62 -2.73 0.68 1.53 1950 1.10 0.49 -1.54 1.50 3.26 -2.79 0.54 -0.51 -2.12 1.83 1960 -1.88 0.47 -1.05 -2.16 -1.04 -1.77 -2.54 1.67 -2.97 -2.58 1970 -1.08 -0.08 2.05 0.27 2.18 0.01 0.59 -2.08 -0.42 -0.23 1980 -1.58 -0.55 2.63 0.38 2.17 -3.09 3.14 -3.01 -0.13 2.38 1990 3.88 1.14 2.54 1.71 2.86 -1.11 -2.00 -0.93

22. Monthly NAO during the 1950shttp://www.ldeo.columbia.edu/NAO/ 1950 0.2 1.9 0.1 2.0 -3.0 0.3 0.5 2.4 4.4 1.6 -0.6 -2.0 1951 1.2 1.7 -2.6 -2.0 -2.6 -3.5 0.0 2.8 0.5 2.1 -0.2 3.8 1952 2.2 -1.6 -2.3 2.8 -2.7 -0.3 0.6 -0.9 -3.7 0.1 -2.8 -1.7 1953 -1.4 0.6 0.5 -3.2 -1.5 1.6 2.6 2.9 2.1 2.6 3.6 1.7 1954 0.0 1.9 0.9 0.8 -2.2 1.1 4.2 -0.2 2.5 1.8 3.5 2.2 1955 -2.5 -2.9 -3.2 1.1 -0.2 -0.3 2.3 2.2 5.4 -2.1 -4.0 -1.1 1956 -1.2 -2.9 0.8 -2.0 6.3 0.5 -1.6 -2.2 -1.2 1.3 2.4 2.3

23. Viewing the NAO • Best to look at westerlies and upper level heights • compare anomalies • westerlies above (+) or below (-) normal • position of upper level high/low systems • Large Negative NAO example • Jan 1956 Great Atlantic Low • Huge upper level ridge over Greenland and Labrador • Strong trough off southeastern US • Rex Block is another sign.

24. Low NAO Storm Example

25. NAO and HistoryMedieval warm period • Medieval warm period (800-1200 AD) • Vikings and Irish Monks traveled the North Atlantic • Vikings • settled Iceland and Greenland • Farmed on Greenland and raised sheep • went to the Islands we now know as Labrador • obtained wood for ships in New Found land • sailed on low sea ice and not very stormy seas • Warmest summers in 8000 years

26. NAO Detection from Past • Wine records • dates of grape harvests • late harvests, cold summers • early harvests, warm summers • location of vineyards…into Scotland • Wheat/Barley Famines • 800-1200 AD wheat grown higher latitudes and altitudes in Europe • barley grown in Iceland (not today!) • bad wheat crops lead to famines • Tree rings and ice core samples

27. Medieval warm period ends • Cold weather began to return slowly in 11th Century • some cold years; such as 1258 • probably related to volcanism • most of the time it was quite warm • 1258 was probably a low NAO year based on tree rings in the region being narrow showing slow growth

28. The Little Ice Age begins • Colder weather began to dominate in 12th century • 1309/10 Thames river freezes with cold and dry weather • tree rings were narrow suggesting cold and dry • NAO < 0 • 1312 very wet; failed wheat crops (NAO>0) • 1315 very wet for next 3 years • extraordinary tree ring growth in Iceland and Ireland • very wet • lead to crop failures and the Great Famine • some countries lost 5-10% of population in large towns • 1320 NAO went negative and ended famine periods and wetness-Record cold set in

29. The Little Ice Age1300 to ~1850 • General period was colder than today by 0.3C. • Some particular cold decades about 0.5C colder • virtually all linked to prolonged negative NAO • warmer intervals linked to positive NAO • some periods linked to: • Maunder minimum - low to no sunspots observed globally • Large volcanic eruptions • Cold in Europe was linked to cold in North America • America became settled in this time frame

30. The Little Ice AgeColdest Decades • 1557-1600 • 1590 were coldest period • 1588- high NAO storm time…Spanish Armada.. • 1690s • heart of Maunder sunspot minimum • 1810s

31. Identifying the Coldest Decades • Ice cores cover the entire period • Tree rings cover the entire period (>300 locations) • Volcanic dust layers • Reliable wine harvest data present to ~1400 • Other records • local harvest and famine records • bad wheat harvest ~ high NAO index wet years • writings and art work

32. Other NAO impacts on Climate • Volcanism • 1600 was notable year • Sunspots • Sporer minimum 1460-1550 • Maunder minimum 1645-1715 • period of prolonged low sunspot activity • 1672-1704 no sun spots were observed • correlated closely to cold period • many low NAO years • 1810 was a no sun spot year • sun spots may be proven to impact Climate.

33. Pacific North American Patten • Flow Pattern over North America and adjacent Pacific Ocean • referred to as the PNA • . The standard is from Wallace and Gutzler (1981) where they defined the index as: time(year)=[z(20N,160W)-z(45N,165W)+z(55N,115W)+z(30N,85W)]/ 4

34. PNA Description • The PNA pattern is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extra-tropics, appearing in all months except June and July. • The PNA pattern reflects a quadripole pattern of height anomalies, with anomalies of similar sign located south of the Aleutian Islands and over the southeastern United States. Anomalies with sign opposite to the Aleutian center are located in the vicinity of Hawaii, and over the intermountain region of North America (central Canada) during the winter and fall/spring.

35. PNA Plots (Nov-Mar)

36. Positive PNA-January

37. Negative PNA-Januaryhttp://www.cdc.noaa.gov/USclimate/pna.html

38. PNA Quadripole

39. Conclusions • The North Atlantic Oscillation (NAO) • Impacts the seasonal weather over eastern North America, western Europe and Greenland • Has two phases in the cold season • The Pacific North American Index (PNA) • Describes the primary mode of atmospheric circulation pattern across North America and adjacent oceans. • Related to teleconnections (Aleutian and Southeast) and (Hawaii and Northern Rockies)

40. Web resources • NAO • http://www.ldeo.columbia.edu/NAO/ • http://www.cru.uea.ac.uk/tiempo/floor2/data/nao.htm • http://www.cpc.noaa.gov/data/teledoc/nao.html • http://www.cdc.noaa.gov/USclimate/pna.html

41. References Rogers, J.C., 1997: North Atlantic storm track variability and its association to the North Atlantic Oscillation and climate variability of Northern Europe. Journal of Climate10(7), 1635-1647. Hurrell, J.W., 1995: Decadal trends in the North Atlantic Oscillation and relationships to regional temperature and precipitation. Science269, 676-679. Wallace, J.M. and David S. Gutzler, 1981:"Teleconnections in the Geopotential Height Field during the Northern Hemisphere Winter" Mon. Wea. Review,109,784-812. Teleconnections Linking Wolrdwide Climate Anomalies. ed. M.H. Glantz, R.W. Katz and N. Nicholls, Cambridge University Press, 1991. Rogers, J.C. and H. Van Loon, 1979: "The Sea-Saw in winter temperatures between Greenland and Northern Europe. Part II. Some atmospheric and oceanic effectes in middle and high latitudes." Mon Wea. Rev. ,107, 509-519.